Mars Oppositions

Mars Oppositions occur when planet Earth, on its inner orbit, passes between
the Sun and planet Mars. Because of the orbits of the two planets, this occurs
about every 2 years and 2 months (26 months); the acurate average is the
synodic period of 779.94 days. After that period of time, the Earth has moved
around the sun exactly one time more than Mars, or 2.135311456 times, while
Mars completed 1.135311456 revolutions, so that Earth and Mars are again in a
line from the Sun. This line, however, is advanced at an angle of 48.71212423
degrees along the planetary orbits w.r.t. the previous opposition, i.e., in
ecliptical longitude.

About at these times, the planets have their closest encounter, but because of
the orbital geometry (excentric elliptical orbits and orbital inclination),
the exact dates may be separated by a few days. A further disturbation comes
from the influence of the Moon which makes Earth move around the common
center of mass, and makes it being a bit early or late, a bit closer or more
remote than just this center of mass.

Also because of the orbital geometry, in particular the comparatively high
excentricity of Mars' orbit and the resulting variations in its orbital and
angular velocity, the intervals between consecutive oppositions vary, perihelic
oppositions (when Mars is near its perihelion, resulting in close approaches)
are less frequent than aphelic oppositions (with Mars near its aphelion, less
close encounters).

After 7 oppositions or about 15 (actually 14.95) [Earth] years (7.95 or almost
eight Martian revolutions or Mars years), this angle has summed up to 340.985
degrees, so that the 7th opposition is only 19.0 degrees in ecliptical
longitude away from (behind) the original one, and occurs about 19 [Earth] days
earlier in that year. After 15 oppositions or about 32 (32.03) [Earth] years
(17.03 Mars years), the opposition will occur 10.6 degrees after the original
one, and happen about 11 days later in that year. After 22 periods (46.977
Earth years, 24.977 Mars years), the opposition will happen 8.3 deg or 8.5 days
before the original date. After an interval of 37 synodic periods (79.0065
Earth or 42.0065 Mars years), opposition will occur only 2.35 deg or 2.38 days
after the original one, less than one percent of the revolution angle (of 360
deg). Another close hit will occur after 96 synodic periods (204.990 Earth or
108.990 Mars revolutions) when the opposition occurs 3.63 deg or 3.69 days
before the first opposition date. Even closer hits will be after 133 synodic
periods (283.996 Earth years, 150.996 Mars years) at 1.29 deg or 1.31 days
before the first opposition date, and after 170 periods (363.0029 Earth and
193.0029 Mars years) at 1.06 deg, 1.08 days. Eventually, after 303 synodic
periods (when Earth has moved 646.9994 times and Mars 343.9994 times around the
Sun), opposition will occur only 0.226 degrees or 0.229 days (5 1/2 hours!)
earlier than the originally considered opposition.

The following table summarizes these results (note that the number of
oppositions is always the difference between Earth and Mars revolutions, or
"years", by definition):

Closest Approaches of Mars to Earth

Mars oppositions are closest if the red planet is close to its perihelion when
it comes into opposition to the Sun from Earth. Closest approaches can become
closer than 56 million km at these times, if the perihelion at its heliocentric
eclipitcal length of 336 deg is hit well.

Very close encounters of Mars to Earth happened in Halley's and Newton's time
in 1687 and 1719, in Messier's time in 1766, in the time of John Herschel in
1845, and in 1924 when galaxies had just been recognized by Hubble as island
universes. The latter two (1845 and 1924) have already been millennia records
each, as is the close encounter of 2003.

Following is a list of all close encounters when Mars has approached, or will
approach Earth closer than 56.00 million km ..
during the first 3 millennia A.D. (1 AD to 3000 AD).

Oppositions at least as close as the 2003 opposition are printed bold.
New record approaches are marked with exclamation marks.

The orbit of planet Mars is subject to small and slow periodic and secular
changes caused by various perturbations, in particular the gravitational
perturbations by the other planets. One of these effects is that the
excentricity varies over the millennia, and thus the perihelion and aphelion
distances of Mars are subject to small and slow changes. Currently, the
excentricity is minutely growing, so that the perihelion distance of Mars is
getting a bit smaller and the aphelion distance a bit larger:
In 4000 B.C. it was about 0.088, today (about 2000 A.D.) it is about 0.093,
and in 6000 A.D., it can be predicted to be about 0.097.
This results in a small decrease of the distance between Earth and Mars in
perihelion oppositions (and equally small increases for aphelion oppositions),
so that records in close approaches will occur in the upcoming centuries.

[Analogous variations of Earth's orbit make the Earth's excentricity currently
decreasing, from 0.019 in 4000 B.C. over today's 0.017 to a mere 0.016 in 6000
A.D.]

Within a period of 1 million years, from 500,000 B.C. to 500,000 A.D., the
perturbations mentioned above lead to oscillations of the possible minimal
distances at perihelic oppositions, between below 54 million km, and more than
64 million km.

The last approach closer than that of August 27, 2003 occurred on
September 24, 57,617 B.C. at 55.718 million km (0.372452 AU) when Mars
showed up at an apparent diameter of 25.15".

It was even smaller and reached an absolute minimum in the last 500,000 years
on January 22, 78,957 B.C. at 53.667 million km (0.358739 AU). Mars was (or
could have been) seen as big as 26.11".

The next grand minimum will occur on May 4, 25,695 A.D. at 53.821 million km
(0.359769 AU), with Mars at 26.04".

The absolute minimum in the upcoming 500,000 years (and the overall million
years under consideration) will be reached on September 20, 294,851 A.D.,
when the planet will come as close as 53.637 million km (0.358541 AU) and
occur under an angular diameter of 26.13". (Wow!)